The evolution of wireless communication systems has followed a sequence of “generations”, from first generation analog systems and second generation (2G) digital systems that were mainly focused on providing circuit switched voice services, via third generation (3G) systems that were capable of also providing moderately high data rate packet switched services, to the current fourth generation (4G) systems in which all services are provided in terms of packet data services. A widespread 4G standard is the third generation partnership project (3GPP) long term evolution (LTE). The work of defining a fifth generation (5G) wireless communication standard is very comprehensive and a future 5G standard should support a variety of different use cases such as mobile broadband (MBB) with massive multiple input-multiple output (MIMO) radio link support, low latency, high reliability communication, low cost machine type communication (MTC) as well as low power Internet of things (IoT) wireless communication devices.
In many of these 3GPP systems a cellular wireless communication device without any ongoing or recent data transmission is said to be in idle mode. In idle mode, the wireless communication device can perform random access to establish a connection to the network. The wireless communication device can also be paged by the network, whereby the network requests the wireless communication device to establish connection with the network for, e.g., an imminent downlink data transmission.
To enable random access, the wireless communication device must keep track of the cell it would use as its target for random access. This process is called cell selection or cell reselection. Cell selection/reselection are examples of mobility processes that can be performed by the wireless communication device. Another mobility process is a handover process where a wireless communication device, not necessarily being in an idle mode, finds that it must switch from communicating with a first cell to continue communication in a second cell, e.g. when traveling along a geographical path. In the following, reference will mainly be to the mobility process of cell reselection, although the issues discussed are similar in mobility processes such as cell selection and handover.
Based on synchronization signals transmitted from the different cells, the wireless communication device determines which cell is best, and the wireless communication device is said to camp on that cell. Once the wireless communication device has determined the best cell, it reads the system information for that cell, and with that information, the wireless communication device can perform random access.
When the wireless communication device geographically moves through the cellular network, it constantly reselects new cells. Once the wireless communication device detects that a synchronization signal is better than that of the cell on which the wireless communication device currently camps, reselection is made to the better cell. When the wireless communication device reselects a cell on the same carrier frequency, this process is called intra-frequency cell reselection.
The cell reselection process is performed with no or very little assistance from the network. Thus, the wireless communication device can find synchronization signals corresponding to new cells blindly or almost blindly. In particular, the wireless communication device searches blindly over a time window for any occurrence of these synchronization signals.
The wireless communication device's ability to blindly reselect a new cell builds upon the fact that in legacy systems, each cell is required to constantly transmit synchronization signals that the wireless communication device uses to identify neighbor cells as possible reselection targets. These synchronization signals are frequently transmitted, e.g., every 5 ms. By recording the signal received for a little more than 5 ms, the wireless communication device can capture all synchronization signals transmitted from close-by neighbor cells. By subsequently processing the recorded signal off-line, the wireless communication device can detect all relevant reselection target cells. Since the synchronization signals are transmitted quite frequently, the wireless communication device can activate its radio receiver circuitry for a short period, leading to low power consumption in the wireless communication device. Also, since the recorded signal is rather short, the amount of memory required to store the recorded signal is rather small.
In legacy systems, when the wireless communication device cannot find any cell above a certain quality threshold on the same carrier frequency, the wireless communication device starts to search for neighbor cells on other carrier frequencies. This process is called inter-frequency cell reselection. The wireless communication device searches for neighbors on other carrier frequencies for some time. If it cannot find any cell on other carriers, the wireless communication device starts searching for cells with other radio access technologies (RATs), to perform a so called inter-RAT cell reselection.
For 5G cellular systems, it is likely that synchronization signals will be transmitted more sparsely, e.g., every 100 ms. Having a larger interval between the idle mode synchronization signals is crucial for the base station energy consumption.
The state-of-the-art solution to cell reselection is essentially turning the radio receiver on to record the signal received for a period that is slightly larger than the synchronization signal transmission interval, then switching the radio receiver off, and subsequently applying off-line processing on the recorded signal. For example, in an LTE system, the wireless communication device stores, e.g., 6 ms (noting that the synchronization signal distance in LTE is 5 ms) and correlates towards the primary synchronization signal (PSS) to find a correlation peak indicating a timing for a cell, and then at that timing correlates toward all secondary synchronization signal (SSS) sequences to find the best match. Then optionally that cell identity corresponding to the best SSS match is used to correlate to cell-specific reference signals (CRSs) (reference symbols) in order to verify the detection. If the reference signal received power (RSRP) is greater than a threshold, then the cell is determined to be present.
However, using the state-of-the-art solution with a large synchronization signal transmission interval requires the radio circuitry of the wireless communication device to operate for a long period, causing high energy consumption. Additionally, since the recorded signal is quite long, a large amount of memory will be required to store the recorded signal for subsequent off-line processing.
With regard to the mobility process of handover, similar drawbacks exist. Relying on state-of-the-art solutions for measurements on other cells would lead to that the wireless communication device would have to search for synchronization signals in a quite large interval. This would lead to similar effects on the wireless device power consumption and complexity.